Multiple source vibrator technology has been used in land-based seismic surveys for years, and its advantages are well documented. Conventional land-based seismic assays employ multiple, simultaneously energized seismic sources (e.g., trucks with vibrating baseplates) to impart vibratory energy into the ground. The imparted vibratory energy travels through the ground, is reflected and/or refracted by various discontinuities in the ground, and the reflected vibratory energy is detected by multiple seismic receivers (e.g., geophones) that are located on the ground at a distance from the seismic sources. The reflected vibratory energy recorded by the receivers is a composite reading representing the reflected energy originating from all the seismic sources. An important step in conventional multiple source vibrator technology is “source separation” of the composite data into discrete source specific data.
In order to allow for source separation, conventional multiple source vibrator technology requires multiple sweeps to be performed while the seismic sources maintain a fixed location. According to conventional multiple source vibrator, the number of sweeps performed at a fixed source location must be equal to or greater than the number of sources sought to be separated. For example, if four seismic sources are being used in a conventional multiple source vibrator acquisition scheme, at least four sweeps must be carried out for each source set-up. In addition, conventional multiple source vibrator technology requires uniquely encoded (e.g., phase, frequency, and/or amplitude encoded) vibratory energy for each sweep, so that source separation of the resulting composite data can be performed.
Marine data are traditionally collected using multiple air-gun sources with multiple streamers of receivers. Multiple sources mean flip-flop shooting, decreased fold, aliased multiples and other undesirable acquisition patterns. Multiple streamers mean increased minimum-offset for some bins and increased noise from the diverging side wings for other bins. Single-source single-streamer acquisitions offer much better sampling, but with such low productivity (in terms of square kilometers acquired per day) that 3D acquisition costs become prohibitive.
Although multiple source vibrator techniques have many known advantages, multiple source vibrator techniques have not been applied to marine seismic surveys for a variety of reasons. For example, the most commonly used seismic sources for marine seismic acquisition (e.g., air guns) are not capable of generating the uniquely encoded vibratory energy required for later source separation. Perhaps a larger obstacle to the implementation of multiple source vibrators in marine environments is the requirement that multiple sweeps be performed with the sources located in a fixed position. In marine environments, it is nearly impossible to perform multiple seismic sweeps with multiple seismic sources remaining in a fixed position, particularly in light of the ocean currents and winds common to most marine environments. Finally, the marine environment has unique reflectance properties that can interfere with the encoded vibratory signal.
What is required are methods of obtaining high fidelity seismic surveys with continuous and/or multiple sources simultaneously in a marine environment using uniquely encoded vibratory signals.